This invention relates to the nondestructive measurement of the fractions of phases present in composite materials, and, more particularly, to calibration relationships and specimen preparation techniques useful in such measurement.
Composite materials are mixtures of two or more phases in which the components retain their inherent character within the mixture. Such materials can, when properly engineered, provide properties that are superior to those that could be obtained from the individual phases acting alone. In just twenty years, advanced composite materials of high performance fibers within a nonmetallic matrix have become standard materials of construction for aerospace and other applications, because of their superior strength-to-weight and modulus-to-weight properties.
Composite materials have the additional desirable feature of permitting the material of construction of a structure to be tailored precisely to the requirements of the structure, and even to the requirements of different parts of the structure. For example, different regions of a composite aircraft wing can be made from different materials to meet modulus or strength limitations, as needed. The fractions of the phases in the different regions can also be varied as needed, as can the arrangement of the fibers. The designer of a composite structure has more degrees of freedom in specifying the exact nature of the material of construction than possible with non-composite materials, and can therefore more precisely meet the needs of the design.
When composite materials are fabricated or used, there is generally no question as to the nature of the constituents. There can be, however, some doubt as to the fractions of the phases present. The machines that manufacture the nonmetallic composite materials in a form known as a "prepreg" usually supply a known amount of fiber material per unit time in a continuous manner, and the nonmetallic matrix material is infused around the fibers. One objective of the manufacturing process is to supply an exactly controlled amount of fiber material and matrix material, thereby exactly determining the relative fractions of the phases. However, with present manufacturing technology, there is inevitably some variation in the amounts added, leading to variations in the fractions of the phases present.
Aerospace designers have developed acceptance standards requiring that the fractions of the phases be supplied to within specific tolerances, currently +/-2 to 3 percentage points. For example, the designer may specify that a composite material include precisely 36.4 +/-2.0 percent by weight of resin matrix phase. If the manufacturer supplies a composite material having less than 34.4 or more than 38.4 percent by weight of matrix phase, the material is rejected. For some applications, even these tolerances are too large, and in those cases the designers would prefer tolerances of as small as +/-0.5 percentage points.
Because of the need to known the amount of phases present to be certain that they are within the specified standards, it is critically important to the manufacturers of composite materials that they be able to measure the fraction of the phases present in the prepreg composite material during manufacture to ensure that unacceptable material is not shipped to customers. It is equally important to the users of the prepreg composite material to be able to independently check the fraction of the phases present in the material received, to be certain that the acceptance criteria are met.
For years, the standard approach to determine the fractions of the phases was to excise a piece of the composite material, weigh the piece, dissolve or burn away the matrix, and weigh the residue that was assumed to be the fiber material. This approach suffers from several drawbacks, including its destructive character, lack of reproducibility, and use of noxious chemicals or production of noxious gases during removal of the matrix. This technique is also quite slow and expensive, and is not adapted to feedback control of the manufacturing operation.
To avoid these disadvantages, a nondestructive ratioing approach was developed. The thickness of a working specimen is measured, and the fraction of the phases determined as the ratio of the thickness of the working specimen to a previously measured baseline specimen, times the destructively measured phase fraction of the baseline specimen. However, thickness of a piece is not a material property. The results obtained with this technique are therefore not meaningful, unless it is assumed that the areal weight of fiber in the baseline specimen is the same as in the working specimen, an assumption that can sometimes be valid in specimens taken from a manufacturing machine whose operating parameters are not varied in the slightest, and therefor the fiber content is precisely the same for the specimens. The assumption is often not valid in manufacturing operations, and is valid only fortuitously for composite materials from different lots and/or different manufacturers. The operability of this technique is thus dependent upon assumptions about the results, an undesirable situation.
More recently, a nondestructive technique preferably using ultrasonic measurements and a calibration relationship was introduced, see U.S. Pat. No. 4,794,545 and allowed Application Ser. No. 07/290,810, now U.S. Pat. No. 4,961,346, whose disclosures are incorporated by reference. A refinement to the technique improved its accuracy, see U.S. Pat. No. 4,856,335, whose disclosure is incorporated by reference.
In the preferred approach according to these patents, an ultrasonic wave is passed through a calibration specimen and some property such as ultrasonic slowness determined. The fraction of the phases is measured by the destructive measurement approach, and the ultrasonic measurement correlated with the destructively measured phase fraction. At least two, and preferably a large number, of the calibration specimens are measured, and the results combined as a calibration relationship. Then the same ultrasonic property is measured on a working specimen, and the phase fraction determined from the calibration relationship. This technique has been verified on a large number of composite material systems, and provides measurements of phase fractions accurate to within +/-0.5 percentage points, or even better, for many composite materials systems.
However, some composite materials systems have been identified wherein the measurements are not consistently accurate to within the required limits. Moreover, in those systems wherein the required accuracies can be achieved, it would be desirable to achieve even better accuracies. Thus, there is a continuing need for a further improved methodology for analyzing the fractions of the phases of composite materials by nondestructive procedures. The present invention fulfills this need, and further provides related advantages.